R208-015 fix Possible_Values_Of for aggregated projects

When tying to get possible values of a Scenario Variable
pass the node tree reference for the actual aggregated subtree,
not the one from aggregate root.
For that to happen without changing API we need to store it
in the SV itself.

Change-Id: Ic551ffce95d5a602ee61317c98e3c3751a2287f3